A woman has told of her traumatic ordeal as she lay pinned under rubble on a Christchurch bus, which had been crushed by a collapsing building during the earthquake of February 22.

Ann Brower was giving evidence this morning at the Royal Commission of Inquiry into the collapse of two buildings on Colombo St, one of which killed four pedestrians and the other which crushed eight passengers on the Red Bus.

On February 22, Ms Brower was reading a magazine on Red Bus Company bus No. 702, which was travelling north along Colombo Street, across from 605 Colombo St.

Suddenly, the magnitude 6.3 earthquake hit and a large amount of masonry and bricks from the building at 605-613 Colombo St fell onto the bus.

"The bus started shaking violently. It stopped at the first jolt. I looked out the window to the eastern side of Colombo St and saw bricks falling off the buildings.

Within a second or two I heard bricks falling on the roof of the bus. The next thing I remember is passing out,'' she told the commission today.

When she came to, Ms Brower was trapped in a safe pocket of the bus, but the bus' roof had collapsed on her, pinning her down. Around her were the dead bodies of seven other passengers.

She was badly injured.

"Although I was still on my seat I was pushed forward so that I was leaning on my legs, I was twisted around, and my left knee was wrenched around.''

Rescuers attempted to keep Ms Brower calm while they tried to free her.

"They just wanted me to stop screaming, apparently I just kept screaming and it was disturbing the rescuers. I remember they told me stories about fishing,'' she said.

"It felt like more weight was coming on to me and I wasn't sure how much I could take. Later I found out that it was because they were actually taking weight off me and more blood was flowing to my broken leg.''

Ms Brower told rescuers to free others first because she was sure she was going to make it. But most were already dead.

"I remember someone saying "Mate get her first, he's gone.''

Ms Brower was eventually freed after the roof was lifted from the bus and she was rushed to hospital.

Her leg was badly broken with the tibia bone sticking out through the skin, her pelvis was broken, she had received a blow to the head and a cut had severed a tendon in her hand.

"I passed out several times from the pain.''

She stayed in hospital until April recovering from her injuries. Several of those who had helped her on February 22 came to visit her, and said they had tried desperately to rescue others on the bus.

"They told me they did their best and really wanted to save them, but couldn't.''

The building at 605- 613 Colombo St, which collapsed on the bus, was green stickered following the September earthquake. The Boxing Day aftershock caused much more significant damage to the building, and it was red-placarded.

Consultation between the owner and its engineers resulted in a decision to demolish the building but, because the building had a heritage classification, the city council required that the demolition application be a notified consent application. This process would have taken some months.

There was no protective fencing in front of the building at the time of the February earthquake.

Ms Brower, a lecturer specialising in regulations, said: "There's some irony in the fact that it was a failure in regulations that nearly killed me and which did kill those closest to me [on the bus].''

Today's hearing will also hear evidence into the collapse of the facade of the building at 603 Colombo St which killed Joan Weild, 76, and Graham Weild, 77, and Israeli backpackers Ofer Levy, 22, and Gabi Moshe Ingel, 22, who were walking in the street nearby.

The heritage classified building was severely damaged following the September earthquake and was yellow-stickered. Following Boxing Day, it was red-stickered. Make safe works were to be completed before January 31 2011.

But no make-safe works were completed on the building and instead a decision was made to demolish it. As this building was also heritage classified, the process involved a notified consent, which was to take months.

The hearing will continue this afternoon, with engineers, building owners, and a city council representative giving evidence. It is expected to conclude tomorrow.